As is generally known to persons skilled in the art, a profilometer is an instrument that performs form and roughness measurements on the outer surfaces of parts, workpieces, micro-scale articles and other types of objects. Typically, the profilometer includes a mounting component for holding an object while it is measured, a probe for measuring the shape of the object, and a motorized drive system for moving either the probe or the mounting component and object. The probe is usually either a mechanical instrument that contacts and traces a path over the outer surface of the object, or is an optical, non-contact instrument that scans the outer surface of an object. The probe is highly sensitive to deviations in the contour of the object being measured. The output signals produced by the probe during measurement are transmitted to an electronic processing unit, which conditions, amplifies, and otherwise processes the signals as appropriate to provide easily interpretable measurement data, surface plots, and/or any other useful output information.
The operation of profilometers has traditionally been based on Cartesian geometry, i.e., a two-dimensional X-Y or three-dimensional X-Y-Z rectangular coordinate system. However, the measurement of objects having large aspect ratios such as, for example, high-sag aspheric lenses, presents a significant challenge for commercially available Cartesian-based profilometers. It is known, for instance, that optical profilometers are slope-limited to a few degrees by the ratio of fringe spacing to resolution. Mechanical profilometers are typically limited by the clearance angle of their probe tips and the non-perpendicular loading direction of the probe, both of which often limit the measurable slopes to less than 45 degrees.
As an example of the limitations of conventional profilometers, FIG. 1A illustrates a probe 12 of a Cartesian-based mechanical profilometer measuring a hemispherical object 14 mounted to a flat surface 16. The object 14 has an arcuate outer surface 14A with a large range of slopes. The direction through which probe 12 traverses when tracing arcuate surface 14A is limited to the X-axis, and the direction through which probe 12 is deflected to sense the profile of arcuate surface 14A is limited to the Y-axis. It can be observed that measurements taken by probe 12 will be incorrect when its clearance face 12A, rather than its tip 12B, contacts object 14. By comparison, as shown in FIG. 1B, if probe 12 could be rotated through an angular direction θ so that its sensing direction R is radial and tip 12B remains in contact with arcuate surface 14A, accurate measurements of arcuate surface 14A could be performed.
Many precision-fabricated parts for which accurate profile measurement is desirable exhibit geometries that can be characterized as being more polar than Cartesian in nature, particularly objects having highly sloped outer surfaces such as arcuate surface 14A illustrated in FIGS. 1A and 1B. Examples include high aspect-ratio, aspheric optical components manufactured by the defense, information technology, and consumer products industries. Currently available Cartesian-based profilometers cannot measure such objects with a sufficiently high degree of precision. It would therefore be advantageous to provide a profilometer capable of effecting measurements of the outer surface of an object that are based on a polar coordinate system instead of the conventional Cartesian coordinate system.